1. Field of the Invention
This invention relates to a technique for charging raw materials, carbonaceous materials, and the like into a moving hearth furnace. More particularly, this invention relates to a material charging method and apparatus for use in a moving hearth furnace wherein raw materials comprising metal-containing materials such as ores containing metals, e.g., Fe, Cr and Ni, iron manufacturing dust and sludge, and industrial wastes (collectively sometimes hereinafter referred to as a xe2x80x9craw materialxe2x80x9d), are charged/deposited on a moving hearth along with carbonaceous materials comprising solid reductants such as coal, coal char, and coke (collectively sometimes hereinafter referred to as a xe2x80x9ccarbonaceous materialxe2x80x9d), and then the deposited raw material is heated for reduction and melting during movement of the moving hearth within a heating furnace, thereby recovering reduced metals continuously. The method and apparatus of the invention reliably and easily performs production, separation and discharge of reduced metals after melting.
2. Description of the Related Art
A reduced metal, e.g., steel, is generally produced by a converter or an electric furnace. According to the process using an electric furnace, scrap and reduced iron are melted under heating with electric energy and then refined, if necessary, for production of steel. Recently, however, there is a tendency to employ reduced iron instead of scrap because of a stringent relation between demand and supply of scrap and an increasing demand for higher quality steel.
As one of the processes for producing reduced iron, etc., the so-called xe2x80x9cmoving hearth furnacexe2x80x9d is known in which an iron ore and a solid reductant are charged/deposited on a horizontally moving hearth, and the iron ore is heated for reduction by radiant heating from above, thereby producing reduced iron (Japanese Unexamined Patent Publication No. 63-108188). The moving hearth furnace used in the above known process is of the type that the charged and deposited raw material is heated during horizontal movement of a hearth within a heating furnace. Usually, the horizontally moving hearth is constructed to move along a ring-shaped locus (i.e., to make a revolution) as shown in FIG. 1.
A rotary hearth furnace will be described below as one example of moving hearth furnaces.
As shown in FIG. 1, a conventional rotary hearth furnace has a ring-shaped furnace body 10 partitioned into a preheating zone 10a, a reducing zone 10b and a cooling zone 10d, which are arranged in that order from the supply side of raw materials toward the discharge side. Within the furnace body 10, a ring-shaped moving hearth 11 is disposed to be able to rotate. A mixture of a raw material, e.g., iron ore, and a carbonaceous material serving as a solid reductant is charged onto the moving hearth 11 under rotation.
It is to be noted that raw material pellets including carbonaceous materials therein are suitably used as the mixture of both the materials. A refractory is placed on the surface of the hearth 11, but a particulate refractory, for example, may be laid on the hearth surface. Burners 13 are disposed in an upper portion of the furnace body 10. By using the burners 13 as heat sources, metal-containing oxides, such as iron ores, deposited on the moving hearth 11 are heated for reduction in the presence of reductant, thereby obtaining reduced iron. Additionally, in FIG. 1, numeral 14 denotes a charging apparatus for charging the raw material onto the hearth, and numeral 15 denotes a discharging apparatus for discharging the reduced material.
Meanwhile, general metal-containing materials, e.g., iron ores, contain many gangue components which are different depending on the places of their production, whereas coal, coal char, and coke as typical solid reductants also contain ashes. In the moving hearth furnace, gangues are unavoidably mixed in the reduced iron having been produced, and ashes contained in the reductants also remain affixed to the reduced iron. This has imposed restrictions that only those raw materials and solid reductants having high-grade quality must be employed in the moving hearth furnace.
Further, if reduced iron containing gangues and ash in large amounts is introduced as a raw material to an electric furnace, the amount of lime to be used for adjustment of the basicity of slag must be increased in the operation of the electric furnace. This necessarily increases not only the cost due to an increase in the amount of lime used, but also the amount of consumed energy, e.g., electric power, due to an increase in the amount of heat required for removing the lime in the form of slag. From that point of view, it has also been essential in the operation of a conventional moving hearth furnace to use only high-grade iron ores containing gangue components as small as possible, and to use reductants containing ashes in smaller amount.
Recently, however, the use of lower-grade materials has been obliged due to exhaustion of resources, such as iron ore and coal, and changes in properties thereof. Thus, this situation has created a problem to be solved as quickly as possible.
Because of those reasons including the necessity of using higher-grade raw materials and carbonaceous materials, there has been a need for development of techniques capable of effectively separating metal components and gangue and other unnecessary components from each other. For example, one method for separating metal components and gangue and other unnecessary components is melting separation of gangues and ash from reduced iron, the separated gangues and ash being treated to turn to slag for removal.
Melting reduced iron on the hearth, however, causes the problem that the melted metal adheres by fusion to the hearth refractory or enters fine cracks or the like and, hence, damages the hearth refractory when the solidified metal is discharged. In particular, because the interior of the moving hearth furnace is subjected to fairly high temperatures for ore reduction, an expensive refractory capable of enduring high temperatures is employed to form the hearth. From the viewpoint of suppressing the production cost of products, therefore, due consideration must be paid so that the hearth refractory will not be damaged for a long period of time.
To overcome the above-mentioned problems, the assignee of this application proposed, in Japanese Unexamined Patent Publication No. 11-106815, xe2x80x9cA method of operating a moving hearth furnace, in which a raw material mainly consisting of a powdery iron ore and a powdery solid reductant is deposited on a horizontally moving hearth to form a layer, and the iron ore is reduced by radiant heating from above within the furnace, the method comprising the steps of depositing a powder mixture of the powdery iron ore and the powdery solid reductant, or a powder mixture of the powdery iron ore, a powdery assistant raw material and the powdery solid reductant on the hearth in the form of scattered small divisions such that the powdery mixture will not be brought into direct contact with the hearth by the presence of the powdery solid reductant therebetween, and then melting reduced iron on the hearth at least once.xe2x80x9d With those features, the proposed method is intended to xe2x80x9cproduce reduced iron containing neither gangues nor ash mixed therein, i.e., reduced iron highly suitably used in an electric furnace, while employing a simple installation and ensuring smooth operation without damaging the installationxe2x80x9d.
However, Japanese Unexamined Patent Publication No. 11-106815 does not specifically describe a method for charging the raw material in the form of small divisions scattered in the carbonaceous material as illustrated in FIGS. 3, 5, 6 and 7 thereof. An intricate process is used to achieve the method for properly charging the raw material in each recess of the carbonaceous material such that the raw material will not come out from the recess and will be kept from mixing with the raw materials charged in the adjacent divisions. The apparatus for charging the raw material is also necessarily complicated. The proposed methods are, therefore, not yet sufficient for practical use in actual operation.
Accordingly, it is a primary object of the invention to provide a method and apparatus for charging a raw material or the like in which a reduced product generated on a hearth can be quickly melted to form small agglomerates with certainty while avoiding the resulting metal and slag to become oversized and, hence, the reduced product can be easily discharged out of a furnace as individual pieces of the small agglomerates.
Another object of the present invention is to establish a technique for producing a reduced metal in a simple manner, while achieving high productivity, without needing any preliminary process for treating raw materials, such as pelletizing them as practiced in the related art.
As a result of conducting studies with the view of realizing the above objects, the inventors have succeeded in accomplishing this invention.
According to one aspect of the invention, there is provided a method of charging a raw material and a carbonaceous material into a moving hearth furnace in which a raw material mainly comprising a powdery and/or granular metal-containing material and a carbonaceous material comprising a powdery and/or granular solid reductant are charged onto a moving hearth to lie deposited thereon, and the raw material is heated for reduction and melting during movement of the moving hearth within a heating furnace, thereby producing a reduced metal, wherein charging the raw material and the carbonaceous material onto the moving hearth comprises the steps of charging the carbonaceous material onto the moving hearth to form a carbonaceous material layer thereon; charging the raw material or a mixture of the raw material and a carbonaceous material onto the carbonaceous material layer to form a raw material layer thereon; and pressing a projection against the raw material layer from above, thereby forming a plurality of recesses on the surface of the carbonaceous material layer.
Preferably, in the step of pressing a projection against the raw material layer from above, a tip end of the projection is depressed to a level lower than a surface level of the carbonaceous material layer, thereby forming a recess that sinks from the raw material layer into the carbonaceous material layer.
Also, preferably, a portion of the raw material layer corresponding to the recess to be formed in the surface of the carbonaceous material layer is depressed at the same time as or before the pressing of the projection.
Further, in the above charging method, the projection pressing step is preferably performed using a denting roll provided with a plurality of projections.
Still further, in the above charging method, before pressing the projection of the denting roll, the raw material layer is preferably depressed in advance by an outer circumferential portion of the denting roll other than the projections.
According to another aspect of the present invention, there is provided an apparatus for charging a raw material and a carbonaceous material into a moving hearth furnace in which a raw material mainly comprising a powdery and/or granular metal-containing material and a carbonaceous material comprising a powdery and/or granular solid reductant are charged onto a moving hearth to lie deposited thereon, and the raw material is heated for reduction and melting during movement of the moving hearth within a heating furnace, thereby producing a reduced metal, wherein the apparatus for charging the raw material and the carbonaceous material onto the moving hearth comprises a carbonaceous material charging unit for charging the carbonaceous material onto the moving hearth to form a carbonaceous material layer; a raw material charging unit for charging the raw material or a mixture of the raw material and a carbonaceous material onto the carbonaceous material layer formed on the moving hearth to form a raw material layer thereon; and a denting device for pressing a projection against the raw material layer from above, thereby forming a plurality of recesses on the surface of the carbonaceous material layer lying under the raw material layer, the denting device being provided downstream of the raw material charging unit in the direction of movement of the moving hearth.
Also, in the above charging apparatus, the denting device is preferably a denting roll provided with a plurality of projections.